Recently, an “HSDPA (High Speed Downlink Packet Access) system” has been specified as a radio communication control system (see, for example, “3rd Generation Partnership Project” on the Internet <http://3gpp.org>). The HSDPA system is a higher speed packet transmission system for a downlink transmission in IMT-2000. The HSDPA system has purposes of achieving a higher peak transmission rate, a lower transmission delay, and a higher throughput and the like.
The HSPDA system is a transmission system for performing communications by sharing one physical channel among a plurality of mobile stations in a time-division. The HSDPA system allocates a channel to a mobile station of better radio quality at each moment. Thus, it is possible to improve the throughput of the entire system.
In order to perform communications by sharing one physical channel among a plurality of mobile stations in a time-division as described above, the mobile stations to perform communication by using the physical channel in each TTI (Transmission Time Interval) must be notified. In the case of the HSDPA system, the notification is made to the plurality of mobile stations by using a shared control channel called HS-SCCH (High Speed-Shared Control Channel).
In this regard, disclosed is a method in which a radio base station sets a transmission power to be allocated to the HS-SCCH, by adding an offset to an A-SPCH (Associated-Dedicated Physical Channel). The A-DPCH is dedicatedly set to each of the mobile stations (see, for example, Japanese Patent Application Laid-open Publication No. 2004-312530). The Japanese Patent Application Laid-open Publication No. 2004-312530 describes that the offset value is controlled in accordance with a communication quality (e.g., a block error rate) of the HS-SCCH. For example, when the required block error rate of the HS-SCCH is 1%, the invention of Japanese Patent Application Laid-open Publication No. 2004-312530 controls the offset value by an outer loop so that the block error rate of the HS-SCCH becomes 1%.
The HS-SCCH is a downlink control channel in the HSDPA, and is a channel for performing a signaling relating to information of the HS-PDSCH (an HS-DSCH as a transport channel) that is transmitted 2 slots after the transmission of the HS-SCCH (see, for example, 3GPP, TS25.211). Here, the information relating to the HS-DSCH includes; an ID of the mobile station to which the HS-DSCH is transmitted, transmission format information, (that is, a channelization code, a modulation scheme) a transport block size, Redundancy Version parameters in H-ARQ, or the like. (see, for example, 3GPP, TS25.212). Accordingly, when the mobile station assigned to the HS-SCCH and the HS-DSCH in the TTI fails to correctly receive the HS-SCCH (i.e., when an error occurs in a demodulation or a decoding of the HS-SCCH), the mobile station cannot receive the information of the HS-SCCH, and hence, does not receive the HS-DSCH.
Meanwhile, since an H-ARQ is performed in the HSDPA, when the mobile station receives the HS-SCCH and the HS-DSCH, the mobile station transmits a decoding result (OK/NG) to the radio base station at a predetermined timing, by using the HS-DPCCH. Here, the HS-DPCCH is an HSDPA control channel in the uplink. Accordingly, when the mobile station fails to decode the HS-SCCH, the mobile station does not transmit the HS-DPCCH at the predetermined timing.
However, since the radio base station has transmitted the HS-SCCH and the HS-DSCH to the mobile station, the radio base station attempts to receive the HS-DPCCH. In such a case, the radio base station determines, with a certain probability, the untransmitted HS-DPCCH as being transmitted, and detects an ACK or a NACK, which are transmission acknowledgment information. When the ACK is detected, the radio base station determines that the HS-DSCH has been received correctly at the mobile station despite the fact that the HS-DSCH has not been received correctly at the mobile station. Then, the radio base station passes the data mapped to the HS-DSCH to an upper layer, and starts a transmission of the next data. This means, in other words, a retransmission in the upper layer occurs.
In other words, when the error in the HS-SCCH occurs, the retransmission in the upper layer may occur. In addition, it is generally known that the retransmission in the upper layer affects a throughput in the HSDPA prominently when a higher throughput rate is achieved.
As mentioned above, according to a method for controlling the transmission power of the HS-SCCH described in Japanese Patent Application Laid-open Publication No. 2004-312530, the transmission power of the HS-SCCH is controlled so that the error rate of the HS-SCCH has a constant value, regardless of being at the edge of a cell or at the center of the cell, or moving at a low rate or at a high rate.
However, there have been problems that the error in the HS-SCCH may lead to the retransmission in the upper layer, and that the retransmission in the upper layer may cause a deterioration of the throughput characteristics in the HSDPA, especially in the environment where the high throughput rate is achievable in the HSDPA.
In view of the above problems, an object of the present invention is to provide a radio communication control system, a radio base station and a radio communication control method that makes it possible to achieve a more stable and higher transmission rate by setting the quality of the HS-SCCH higher, in the environment where the high throughput rate is achievable in the HSDPA.